Answer: It's probably not a good idea to use spikes in the container where the concentrated fertilizer could burn the roots. I'm not sure it was a good idea to leave the pots in place, either. Although you've cut out the bottoms of the pots, the sides will still keep the roots in a restricted area which will probably make them grow round and round, eventually girdling the tree. While building a basin around the trunk of a young tree is a good idea, the roots will eventually travel out past the ends of the branches. By the time the tree has grown a nice sized canopy, the feeder roots will extend about 1 1/2 times the width of the canopy. At this point most trees can get along with just natural rainfall.
Newly planted trees will need a period of adjustment before they're ready to grow, so there's really no need for fertilizer right after planting. Give your trees an opportunity to get settled in. Then, if growth rate is slow or the leaf color is off, use some fertilizer to help it along. Feed in the spring when vigorous growth should take place, but not in the autumn, when new growth won't have a chance to harden off before cold weather arrives. (Tender new growth is quite susceptible to frost.)
Hope this gives you a little insight into the growth habits of trees.
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